Electro-optical incremental motion and position indicator

ABSTRACT

A compact electro-optical system provides electrical signals for indicating extent and direction of incremental movement. By employing a plurality of moire fringe-generating grating pairs in optical series, the system eliminates the need for a collimated light source or imaging optics, and further provides a multiplication of sensitivity.

United States Patent [191 Erickson Oct. 30, 1973 ELECTRO-OPTICALINCREMENTAL MOTION AND POSITION INDICATOR [75] Inventor: Kent E.Erickson, Brookside, NJ.

[73] Assignee: Keuifel & Esser Company,

Morristown, NJ.

[22] Filed: Aug. 17, 1971 21 Appl. No.: 172,383

[52] U.S. Cl. 356/169, 235/92 GC, 250/237 G [51] Int. Cl G0lb 11/04,G06f 7/38, G068 /00 [58] Field of Search 250/237 G, 237R;

[56] References Cited UNITED STATES PATENTS 7/1961 Bower 356/169 OTHERPUBLICATIONS The Possibilities of Moire Fringe Interferometry in 1Interferometry, J. M. Burch, Her Majestys Stationery Office, London1960, QC, 411, T4 Title Page and pages 181, 198, 199, and 200.

Primary Examiner-James W. Lawrence Assistant Examiner-T. N. GrigsbyAttomey-J. Russell Juten et a1.

57 ABSTRACT 17 Claims, 16 DrawingFigures Patented Oct. 30, 19733,768,911

5 Sheets-Sheet l l l 2 2 n n OUTPUT DIRECTION 4% INPUT DIRECTION iPatented Oct. 30, 1973 3 Sheets-Sheet 2 LIGHT a b or INTERMEDIATEDIRECTION I INTERMEDIATE DIRECTION P Patented Oct. 30, 1973 3Sheets-Sheet 1) ff jf ELECTRO-OPTICAL INCREMENTAL MOTION AND POSITIONINDICATOR BACKGROUND OF THE INVENTION Electro-optical systems forindicating minute linear or angular displacement have for some time madeuse of the moire fringe pattern generated by a pair of gratings disposedin a light beam. In many such systems amplitude gratings have been usedwherein each grating in the pair comprises alternating opaque andtransparent areas, usually in the form of regularly disposed parallel orradial lines or rulings. These systems operate in principal upon thefact that relative movement between the gratings in a directiontransverse to the rulings results in a change in the directions of lightoutput from the grating pair. The latter directional change can bevisually observed as a transverse shift in moire fringe position andwhen appropriate optics are employed in the light entering or emergingfrom the grating pair the change in light output directions can betranslated into a generally cyclic fluctuation in the intensity of lighttransmitted by the pair. Photoelectric means are com-' monly used toconvert the varying light intensity to a comparably varying electricalsignal which is employed to trigger electronic counting means in orderto indicate the amount of relative movement between the grating pairelements.

Since the directional perturbations caused by relative movement betweena pair of gratings are indistinguishable by a photocell, it is necessaryto employ optical means to derive moire fringes which vary positionallywith such relative grating movement. Such optics are selected so as toprovide, in conjunction with the desired dimensions of the grating pair,a pattern of moire fringes whose relative position may be readilydistinguished by a photocell, normally as a variation in lightintensity.

Previous systems have employed collimating' optics in the input lightbeam so as to limit the direction of the input light and form a fringepattern at the face of the photocell which has a period of repeatedcycle greatly eitceeding the finite field of view of the photocell,thereby causing a light intensity fluctuation at the photocell as afunction of the positional change in the fringe pattern.

Alternatively, other known systems have employed optics in the outputfrom a grating pair to form a fringe pattern varying in position withrelative grating movement and have additionally employed an aperture tolimit the photocell viewing field to a narrow region of the fringepattern in order to obtain the light intensity fluctuation with thepositional change of the fringe pattern.

In addition to a loss of useful light, these prior devices suffered fromthe disadvantages. occasioned by the size and cost requirements of theincorporated optical elements. Further limitations inherent in previoussystems resulted from the direct relationship between the period of thegrating element pattern and the period of the fringe pattern formed atthe photocell, i.e., the relative movement between grating'elements ofone grating period resulting in a single cycle displacement of thefringe pattern. In order -to increase the sensitivity of these systemsit' was therefore required to provide a greater frequency of rulingswith resulting inordinate expense of grating manufacture. An additionaland more limiting consideration, however, was based upon the fact thatthe spacing between a pair of' gratings must be reduced as the square ofthe grating period in order to retain an effective visibility in thefringes. The necessity of allowing unhindered movement between thegrating pair elements thus severely restricted the reduction in gratingperiodicity.

In order to avoid this grating element spacing restriction, systems weredevised wherein the grating pattern of one element was imaged upon asecond element,

thereby in effect eliminating the spacing between grating elements andallowing a greater reduction in grating periodicity with resultingsystem sensitivity. However, such systems, as described, for example, inU.S. Pat. Nos. 3,244,895 and 3,454,777 represent a compromise of compactdesign by the introduction of additional imaging optics which compoundspace requirements and costs.

The utilization of light signals obtained in prior art systems generallyhas included the use of photoelectric cells to convert the light toelectrical signals. Such electrical signals are employed in eitherdifferential or push-pull common-mode-eliminating amplification systemsin conjunction with electronic counters and display devices. It has beencommon practice to use phase displacement between signal pairs as ameans for eliminating common D. C. signal components as well as toobtain sine-cosine signal sets upon which to base motion directiondiscrimination. Such procedures and related equipment are discussed byBurch (Progress in Optics, Vol. II, Part II, sec. 3.2, ppl l00-l02,lnterscience Publishers, 1963), and various systems are described inU.S. Pat. Nos. 3,244,895; 3,454,777, 3,482,107; 3,483,389; 3,502,414;3,538,339 and 3,573,468.

SUMMARY OF THE INVENTION The present invention provides an'electro-optical system for determining incremental motion and positionchange. The system is generally of the moire fringe type previouslynoted, however, it represents an advancement in the relevant art in thatit employs a simple, non-collimated light source, yet eliminates theprevious need for imaging optics and thereby provides a compact andeconomical measuring device and apparatus. The invention additionallyprovides a multiplication of sensitivity and, by utilizing the compositesignal of a plurality of fringes, provides a high intensity light signaland eliminates the effect of any localized distortions in the elementsof the system.

Whereas previous sytems have provided incremental distance measurementby various means of interpretation of the classic moire fringe patterngenerated by a single pair of ruled gratings, the present invention ischaracterized by the utilization of a plurality of grating pairsarrangedin series in ,the light beam of the system. By such an arrangement thepresent invention makes practical use of gratings which are sufficientlycoarse to be mass-produced by photographic methods, yet provides usefulsensitivity by a multiplication of light signal frequency. Unlikeearlier systems which relied for signal generation upon thefluctuationof light intensity as a function of the position of individual fringesin a moire pattern, thepresent invention provides for the generation ofa fringe pattern which cyclically varies in intensity as a whole. Theresulting intensity fluctuation of the fringe pattern is thusdistinguishable by a photocell detector over a substantial number offringes,

thereby providing an increase in useful signal as well as obtaining anaveraging which eliminates extraneous single-fringe errors.

The grating pairs employed in'the present invention are amplitudegratings which are constructed in the usual manner to provide optimumfringe visibility, that is, each grid element of a pair consists of astable transparent plate having on one surface a set of parallel opaquerulings of a given width, each ruling being separated from the next by aclear transparent space of substantially equal width, thus forming agrid pattern of repeated period, d. It will, of course, be understoodthat gratings bearing radial rulings are to be employed in systemsintended for angular measurement; however, the present discussions willbe limited to linear measuring devices in view of the equivalencebetween the two types of systems.

Each, of the grating pairs in the present invention comprises twosimilar grid elements arranged in parallel planes with their respectiverulings substantially parallel and the elements separated by a space, t,established by the relationship: r'=d where A is the effectivewavelength of the light beam of the system. The wavelength factor willusually vary within a limited range due to the light source selected andthe response band of the photocell used; therefore, while the spacing,t, can be generally calculated, final assembly adjustment is normallyeffected to obtain optimum signal.

When a field of uniform illumination is viewed through one such gridpair arrangement, a typical moire fringe pattern can be seen and isdisposed with the fringe bands running substantially parallel to thegrid rulings. The fringe pattern appears to be located at infinity, theapparent result of the cumulative interference between the adjacentmajor diffraction orders of light entering the grid pair from theinfinitely numerous directions represented by the uniform field, and thepositions of the individual bands of the fringe pattern are locatedaccording to the relative alignment of the rulings of the respectivegrid elements. Relative movement between the grid elements in adirection transverse to the rulings appears as a similar transverseshift in the positions of the fringe bands; however, the relative lightintensities of the bands remain constant.

While the change in direction of light output from the grid pair can beobserved visually as a transverse shift in fringe pattern position, aphotocell viewing a multiplicity of fringes cannot distinguish thechange in direction of light incidence and as a result merely generatesa signal representative of the sum of the intensities of all lightincident upon its face, regardless of the direction of light output fromthe grating pair. Such is the reason that prior systems required opticalmeans to limit the relative width of the photocell viewing field to aportion only of one fringe pattern cycle to allow the photocell to see afluctuation in light intensity and provide a means for counting thenumber of fringes moving past the photocell station.

As a basis for considering the operation of the present invention it cangenerally be stated that a pair of amplitude gratings of the typedescribed can be viewed as having a series of inherent directionalpreferences with respect to the transmission of a light beam in whichthe pair is situated. These directional preferences depend primarilyupon the relative positions of the respective grid patterns of eachelement of the pair and change periodically with relative movementbetween the grid patterns, that is to say, the relative displacement ofone period, d, between grating elements resultsin a shift in directionalpreferences through one cycle, the angular magnitude of one suchdirectional preference cycle, tan d/t. The directional preferences ofthe grating pair may be further defined as comprising a sub-series ofmajor, or preferred," directions varying in azimuth by 0, and a secondsub-series of minor, or nonpreferredf directions bisecting the preferreddirections, I

The directional preference phenomenon of a-grating pair manifests itselfin two basic ways which may be related to the type of input lightincident upon the pair. First, if the grating pair is positioned in abeam of unidirectional, collimated light, the intensity of output lightfrom the pair will appear as a function of the relative alignment ofthat input direction with the directional preferences of the pair;specifically, the output intensity being maximum when the inputdirection is the same as a preferred direction of the grating pair, andminimum when the input is aligned with a nonpreferred direction.

Secondly, if the grating pair is situated in a uniform omnidirectionallight beam, the intensity of output light emerging from the pair in anygiven direction will appear as a function of the alignment of thatoutput direction with the directional preferences of the pair;specifically, the output light being a maximum in the preferreddirections of the grating pair while being a minimum in thenon-preferred directions.

Relative movement between the elements of a grating pair will result, asnoted, in a shift in the directional preferences of the pair and,depending upon which of the two described light input situations exists,will further result in either (1) a change in output light intensity or(2) a change in the directions of output of maximum and minimum lightintensity. In either situation the noted change follows the relativemovement between grating elements and varies through one cycle with eachrelative displacement of grid period, d, between the elements of thepair.

The present invention utilizes the directional preferences of gratingpairs in a measuring system by arranging, in optical series, a uniformfield of illumination, i.e., a source of omnidirectional light input,such as a simple incandescent lamp; two pairs of amplitude gratingssituated in substantially parallel planes; and a photoelectric cell.These elements represent the most simple basics of the system, since, aswill be later dis cussed, a preferred system comprises a plurality ofgrating patterns and photocells to obtain comparative electrical signalsuseful in electronic counters; however, the operation of the presentsystem may be sufficiently considered in terms of the listed elements.

As generally described above, the first pair of gratings, i.e., thatpair upon which the input light is incident, if in fixed relationship,establishes a fixed set of preferred, i.e., maximum intensity, andnon-preferred, i.e., minimum intensity, light output directions. Thesedirections, conversely, represent sets of light input directions withrespect to the second pair of similar gratings which are arranged forrelative movement in the general direction to be measured, as bymounting one grid element of the second pair on a slide moveable in adirection transverse to the grid rulings.

The operation of the present system can be generally understood byconsidering that when the preferred directions of the two grating pairsare aligned, i.e., where preferred output directions of the first pairare the same as preferred input directions of the second, the lightmaximally transmitted through the first pair, i.e., in the preferredoutput directions, will be further maximally "transmitted" through thesecond pair. Concurrently, due to the alignment of the non-preferreddirections of the two grating pairs, the light minimally transmitted bythe first pair in such directions will be further transmitted by thesecond.

Considering now the displacement of the moveable grid element of thesecond pair by an increment of 1/2, one can see that the preferreddirections of the second grating pair have been shifted by an incrementof 0/2, that is to say, the preferred input directions of the secondgrating pair are aligned with the non-preferred output dirctions of thefirst grating pair. As a result of this realignment of directionalpreferences of the two pairs of gratings, the lightrnaximallytransmitted in the preferred directionsby the first pair will now betransmitted minimally by the second pair, and vice versa' Thus it can beseen that the intensity of light transmitted by the combination of twograting pairs is a product function dependent upon the relativedispositions of the directional preferences of each of the two gratingpairs. The photocell of the system effectively integrates thedirectionally-related light intensity products over all directionswithin its field of view and generates an electrical signal whichfluctuates with the displacement between grid elements and the resultingchange in alignment of the directional preferences of the two gratingpairs.

The present invention can therefore be distinguished fromprior singlegrating pair systems which relied upon a changein moire fringe patternposition in response to incremental grid element displacement, by thefact that the present dual pair arrangement creates a change of lightintensity over the whole of the field of view of the photocell with suchdisplacement and eliminates the necessity for means to establish achange in position of a fringe pattern with respect to the photocell.

As will later be noted in greater detail, it is of further advantage toarrange one grid element in each of the two grating pairs for movementwith respect to its mate. The respective directions of movement of theseelements is selected to provide a countercurrent change in thedirectional preferences of the respective grating pairs. In this mannerthe relative rate of change of preferred direction alignments as afunction of displacement is doubled and a doubling of the systemsensitivity is achieved. A simple method of providing countercurrentchange of directional preferences is to fold the light path within thesystem by reflective means. This practice additionally reduces thenumber of physical elements required and yields a one-sided" system,i;e., where light source and photocells are at the same side of thegratings, and provides for ready access for cleaning, adjustment, orrepair.

The prior art is familiar with various electronic arrangements by whichphotocell-generated electrical signals can be counted and displayed. Thecapability of the present invention to produce a plurality of relatedphase-displaced light signals renders it particularly useful inconjunction with counters described in the earlier-referencedpublications. For example, by regularly displacing separate regions ofone grid element pattern in quadrature, there will result separateregions of the output light beam in which intensities may be variedcyclically, but in quadrature displacement. Photoelectric cellsindividually associated with each such region of the output light beamwill provide signals similarly displaced according to phase quadrature,and such signals may be employed in pairs in common-mode-rejectionelectronic systems and the resulting pair of signals may be used toobtain sine-cosine signals for distinguishing displacement direction.

THE DRAWINGS FIG. 1 is a schematic perspective view of a pair ofamplitude grating segments employed in the present invention, showing inparticular establishment of the characteristic directional preferencesof the grating P FIG. 2 is a curve representative of the intensity oflight output from a pair of amplitude gratings as a function of outputdirection;

FIG. 3 is a schematic cross-sectional plan view of the grating pair ofFIG. 1 showing in particular the change of directional preferences as afunction of grid element displacement; I

FIG. 4 is a curve representative of the intensity of light output from apair of amplitude gratings as a function of input direction;

FIG. 5 is a schematic cross-sectional plan view of a combination of twograting pairs according to the present invention, showing in particular,as FIG. 5a and FIG. 5b, a comparison of different alignments of gratingpair directional preferences;

FIGS. 6 and 7 are curves representative of the intensity of light outputfrom two pairs of amplitude gratings as a function of intermediatedirection for the respective positional arrangements of FIGS. 5a and 5b;

FIG. 8 is a schematic cross-sectional plan view of one embodiment of adual grating pair arrangement according to the present invention;

FIG. 9 is a schematic cross-sectional viewof another embodiment of adual grid pair arrangement according to the present invention;

FIG. 10 is a cross-sectional schematic plan view of one embodiment of anentire light signal generating assembly according to the presentinvention;

FIG. 11 is a plan view, partially in section, of a preferred embodimentof the present invention; 7

FIG. 12 is a cross-sectional view of the embodiment of FIG. 11 taken at1212;

FIG. 13 is a sectional plan view of FIG. 12, taken at 13--13 showing apreferred reticle grid pattern;

FIG. 14 is a schematic circuit diagram of an electronic system utilizingthe present invention.

DESCRIPTION OF THE INVENTION The present invention may be described ingreater detail by reference to the drawings where, in FIG. 1, there isdepicted a segment of a pair of amplitude gratings such as generallyemployed in this system. Segments of respective first and second gridelements 1 l, 13 comprise glass plates or other transparent supports ofsubstantial dimensional'stability having opaque rulings or bars 15 attheir facing surfaces alternating with transparent areas 17. The opaqueand transparent areas of the face of the grid elements are ofsubstantially equal width, d/2, thereby establishing a recurring gridperiod of a width, d. Grid elements 11, 13 are set in parallel planesseparated by a distance, t=d/)\; where A is the most significant averagewave length of the useful input of light having an intensity, I,.

At any given time, i.e., when the elements of a grating pair are infixed position, sets of directional preferences for the particular pairare established by the relative transverse dispositions of therespective grid patterns of elements 11, 13. In any grating pair of thetype described, the preferred directions 01,, a or are those directionswhich are parallel to the various straight lines lying in a planeperpendicular to grid elements 1 1, l3 and connecting dissimilar pointsin the respective grid patterns; for example, in FIG. 1, the linesvidentifying preferred directions 01,, a: of the grating pair connect thepoint at the center of a transparent area 17 on grid element 11 with thepoints at the centers'of opaque areas 15 on grid element 13. Conversely,non-preferred directions [3,, [3,, B are established by like points,i.e., the centers of respective transparent areas 17, on the gridpatterns. As indicated, the respective preferred and non-preferreddirections of a grating pair vary by an angle, =tan' d/t, which is theperiod, or frequency of repetition, of each of the particulardirectional preferences.

When a pair of amplitude gratings is placed in a beam of omnidirectionallight of uniform input intensity 1,; the intensity, I,,, of lighttransmitted by the pair will vary according to the alignment between theoutput direction, 45,, and the various directional preferences 0:, B ofthe grating pair. Thus the output intensity, I,,, will be a maximumwhere (b, is aligned with a preferred direction, a, and will be aminimum where d), is aligned with a non-preferred direction, ,8. Thesolid line curve of FIG. 2 indicates the intensity, I,,, of transmittedlight as a function of output direction, 42 for a grating pair when gridelements ll, 13 are in a first fixed position. As can'be seen, asinusoidally varying output intensity results which has a frequency, 6,established by the directional preference period of the grating pair.

If one grid element 13 is displaced in the arrowed direction indicatedin the cross-sectional plan view of the grating pair, FIG. 3, it can beseen that the directional preferences of the pair will be shiftedthrough the period, 0, for each increment, d, of such displacement. Foradisplacement of d/2 the relative preferred, a, and non-preferred, [3,directions will be shifted by 0/2, and the pattern of intensity of thetransmitted light will be located as shown by the dotted line curve ofFIG. 2. This positional shift relates to the characteristic visuallyobserved movements of a moire fringe pattern.

It is apparent from FIG. 2 that the overall intensity of the transmittedlight, i.e., the intensity over a substantial number, n, of periods, 0,as integrated by a photoelectric cell, remains constant. For this reasona photocell is unable to distinguish positional change and incapable ofindicating displacement unless field-restricting optics are employed inaccordance with prior art practices.

Considering now a second situation where, in FIG. I, a unidirectionallight beam of intensity, I,, is incident upon the grating pair; as bycollimation in a given direction, 4),, it will be apparent thatintensity, I,,, of the transmitted light will depend upon the alignmentbetween that direction, 4),, and the directional preferences of thegrating pair. Thus, the more nearly (in is aligned with a preferreddirection, a, of the pair, the greater will be the intensity, I,,, ofthe light transmitted. A continuing change in the input direction, (1),,will therefore effect output intensity, 1,, cyclically in accordancewith the period, 6, of the directional preferences of the grating pair.The curve of FIG. 4 represents the output intensity, I,,, as a functionof input direction, (in; and can be seen to be substantially the same asthe curve of FIG. 2. Similarly, a change in directional preferences, asby displacement of element 13 (FIG. 3), will effect a sinusoidalvariation in output intensity for a given input direction, 42,. Aspreviously noted, a grid element displacement of one period, d, resultsin an output intensity variation through one cycle, 0.

In view of the foregoing, the effect of the combination of two pairs ofgratings arranged in series in an omnidirectional light beam, accordingto the present invention, may be considered. FIG. 5 generally depictstwo positional situations (FIG. 5a and FIG. 5b) of two grating pairs, A,B. In FIG. 5, grating pair A consistsof grid elements 51, 52 such aspreviously described with reference to FIG. 1, and grating pair Bconsists of elements 53, 54. Elements 51, 52, S4 in the presentdescriptive representation are fixed, while element 53 is moveable andmay be displaced in a manner similar to 2 3mm}; nElQ-- The relativepositions of the respective grid elements 51, 52 and 53, 54 establishthe directional preferences of grating pairs A, B, and in FIG. 5a thesepreferences have been arbitrarily set in alignment, that is, a preferreddirection, 0 of grating pair A in this first situation is the same as apreferred direction,

of grating pair B.

In accordance with the previous description, when the dual grating paircombination is placed in a beam of omnidirectional input light ofsubstantially uniform intensity, 1,, the transmitted light output fromgrating pair A, that is, the light intermediate the two pairs ofgratings, as designated by the half-arrow, will vary in intensity, as afunction of the alignment between any output direction, d) and thedirectional preferences of grating pair A. The variation in theintensity of the output light from grating pair A is thus as shown byFIG. 2, and considering, for example, the arbitrarily selectedintermediate direction, 51, it will be seen. since that direction isaligned with the preferred direction, a that light transmitted in thatdirectionas outpuffrom pair A will be a maximum. In view of the factthat the ruling pattern of the grid elements comprises approximately 50percent opaque, light-absorbing areas, the maximum light transmissionthrough grating pair A, or any such grating pair, will be roughlyone-half of input intensity, I,. In this regard also the minimum lighttransmission through a pair of gratings, i.e., in non-preferreddirections, may normally be about 10 percent of the input intensitydueto extraneous scattering of light within a system. Thisrepresentative range of intensities, i.e., between about 50 percent and10 percent, as the maximum and minimum transmissions for a grating pairis employed in the present description, particularly for the purpose ofthe curves in the drawings.

Referring further to FIG. 5 (FIG. 50), it is apparent that the lightoutput from grating pair A is the light input to grating pair B andvaries in intensity, I4, as a function of the direction of output frompair A. Regarding the indicated intermediate direction, 4),, as input topair B, it will be seen that since such input is aligned with apreferred direction, a of the grating pair, the intensity, I,,, of lightoutput from pair B will whims a m. according t hqssryssili fi- 5a, withrespect to any intermediate direction, qS, will thus be the product ofthe intensities of light transmitted by each grating pair as a functionof that intermediate direction; The resulting dual pair outputintensity, I,,, as a function of intermediate direction, 11), wherepreferred directions of the respective pairs are the same, can berepresented by the solid line curve of FIG. 6, wherein the dotted linecurve A, B represents the coincident intensity/direction curves ofindividual grating pairs A and B. The area beneath the product curve inFIG. 6 is representative of the output light intensity as integrated bya photocell in the output beam, the number of cycles, 0, and thus thetotal light incident upon the photocell being determined by thedimension of the field of view of the photocell.

FIG. 5b represents a second positional situation where one grid element53 is displaced in the transverse direction by an increment of d/2,thereby causing a /2 shift of the directional preferences of gratingpair B and alignment of the non-preferred directions of pair B with theunchanged preferred directions of pair A. Considering intermediatedirection, it can be seen that output, 14, from grating pair A is amaximum, qb being a preferred output direction, (1 As input to gratingpair B, however, willbe only'minimally transmitted by pair B, since d),is aligned with nonpreferred direction, B The resulting transmission,In, will, as beforefbe' the product of the intensities transmitted byeach pair in respect of direction, but, due to the displacement noted,will be the product of respectivemaximum and minimum intensities.

The output intensity curve of the dual grating pair arrangement of FIG.5b appears as the product of the intensity curves A,B in FIG. 7, thearea beneath the product curve being total output intensity asintegrated by a photocell. A comparison of the respective integratedintensities, i.e., the areas beneath the curves of FIGS. 6 and 7,provides a representative example of the maxithe grating pairs occurs,thus increasing the frequency of the light signal and the sensitivity ofthe system without a change in grid period, d.

The embodiment represented in FIG. ,8 comprises fixed grid elements 61,65 and'a moveable element 63 situated between the fixed elements andseparated from each by the optimum spacings I t Element 63 carries agrid pattern on each of its faces and these grid patterns form, with therespective patterns on elements 61, 65, the two grating pair's A, Bcharacteristic of the present invention. Displacement of element 63 inthe arrowed direction will be seen to effect a counterclockwise shift, A0 in the directional preferences of grating pair A while simultaneouslyeffecting a clockwise shift, A a in the preferences of grating pair B.This counter-current shifting of the respective directional preferencesof the two grating pairs results in a light intensity signalchang'efrequency, 0, equivalent to a displacement, d/2, of element 63, therebyeffecting a mum and minimum limits of the magnitude of the light signalprovided by an arrangement according to the present invention. It willbe apparent that a displacement of greater or less than d/2 results in alight signal of a magnitude intermediate the represented maximum andminimum and the continuous displacement of one grating element over aplurality of grid periods, d, will effect a quasi-sinusoidal signalhaving a frequency, 0. The frequency of the cycling intensity of thelight signal obtained from a system of the present inventiondeterminesthe sensitivity of the system. As thus far described, such a systemwherein one grid element is displaced is limited in sensitivity by thedimension of the grid period, d, since a displacement of onesuch'period, d, is required to effect a relative shift of one period, 0,in the respective directional preferences of grating pairs A,B. However,by the arrangement of grating pairs as shown in FIG. 8 it is possible toeffect the simultaneous displacement of a grid element in each of thegrating pairs and increase the rate at which alignment and realignmentof the directional preferences of doubling of the sensitivity of thesystem.

An embodiment of the invention as represented schematically in-FIG. 9provides additional advantages and is preferred for its simplicity ofdesign. This-arrangement comprises a single pair of grid elements 91, 93of which element 93 is moveable as indicated; however, due to the factthat element .93 bears a reflecting layer 97 at the face opposite itsgrid pattern, the light beam is folded within the system anddouble-passed through the grating pair, thereby effectively establishingthe two pairs of gratings characteristic of the present invention. Ascan be seen, the displacement of element 93 results in thecountercurrent shift in the directional preferences of the gratingpairs, thus achieving the noted doubling of sensitivity. An advantageinhering in this arrangement derives from the'fact that one spacing, gserves both grating pairs A,B, thereby eliminating differences invisibility of light signal which might otherwise result from animbalance of spacings t t due to accidental wobble or axial movement ofelement 63 in the FIG. 8 arrangement. Additionally this preferredembodiment affords a more compact assembly with the light sour'ce'andphotocells on the same side of grid elements and allows for readyaccessibility to all elements of the system.

A- schematic representation of a complete system of the presentinvention, but for the electronic counting and display means, is shownin FIG. 10 and incorporates the more simply described dual grating pairassembly according toFIG. 8. The system comprises the grid elements71,73,75 which form the two grating pairs; a source 72 ofomnidirectional input light having a width, W,; and a photocell 74having a width W,,, the source and photocell being separated by adistance, D. It should be noted that the diagram of FIG. 10 is not toscale, particularly with respect to the dimensions of the grid patternwhich have been greatly magnified for purposes of clarity. The effect ofspacial dimensions of the 1 system may nevertheless be considered inview of this diagram.

effect of a light distribution function, the intensity of the lightpassing through the system in the region of the longitudinal axis of thesystem will be greater than that incident upon the photocell at theouter limits of the field, 96,; however, since in actual practice thenumber, n, of cycles integrated is relatively large, the distributionfactor is of little consequence. Of more significance, however, is thefact that the effect of any distribution factor remains constant and maybe disregarded entirely, since the ultimate intensity product curveindicative of the light signal varies only in amplitude and not inposition. i

As earlier noted, the present invention is concerned primarily with thegeneration of a light signal which may readily be converted to anintelligible electrical signal for use in known electronic systems anddevices. Therefore, further discussion of the invention would be moreappropriate with reference to a preferred embodiment.

PREFERRED EMBODIMENT A preferred embodimentof the present inventionemploys the one-sided double-pass grating pairs arrangementschematically depicted in FIG. 9. The light signal generating assemblyis shown in greater detail in plan at FIG. 11 and comprises a moveablegrating element of any desired length of which a segment 113 is shownand, set in fixed position above element 113, a multi-patterned gratingelement 111 which will be later described in more detail. Affixed to theupper surface of element 1 11 are a series of photoelectric diodes 201,202, 203, 204 and a simple shed-prism light director 118. Situated abovethis assembly and substantially centered with respect to element 111 isan incandescent light source (not shown). Grating element 113 comprisesa plane glass plate or strip, about 5.0 mm thick and of usual opticalquality, having a regular grid pattern at one face which comprisesparallel lines 115 of black chrome or other opaque material separated byclear areas 117. In this embodiment each of the opaque and clear linesare about 0.008 mm wide; however, these dimensions are not critical, butwere selected on the basis of ease of manufacture by common photoresistand vacuum deposit procedures. Any desired line width may be employed,bearing in mind; however, that such dimension is in part determinativeof the optimum spacing, t, between elements 1 1 1, 1 13. It should alsobe noted that the 1:1 ratio of opaque and clear line widths.

here employed is not critical and, in fact, that a vari-' ance up toabout 25 percent in the ratio can in some instances provide animprovement in light signal. As indicated by the double-headed arrow,element 113 is arranged for displacement in either direction transverseto the grid rulings, measurement of the extent of such displacementbeing the prime object of the invention.

Further detail of the present embodiment may be seen by reference to thecross-sectional view of FIG. 12. In addition to the pattern of gridlines 115 on its face, the glass plate 112 of moveable grating element113 bears a fully reflective aluminized coating 1 14 over substantiallythe whole of its back surface. The multipatterned grid, or reticle,element 111 comprises an optical glass plate 116 of a thickness of about0.6 mm which has a series of grid patterns, some of which are shown at102, 103, 121, on its face. FIG. 13 shows the entire series of gridpatterns which will later be described. Cemented to the back of reticleplate 116 are silicon diode photocells, two of which are shown at 202,203, and the light director 1 18. Element 118' serves merely to cant thebeam of light from source 122 in the general directions of thephotocells and may be of any transparent material, standard opticalquality glass again being selected for the present embodiment. Lightsource 122 is a common incandescent tungsten lamp (G.E. 2124 D) of about0.75 watts and provides a beam of omnidirectional light which serves asinput to the dual grating pair system. The previously noted dimension,W,, may be considered to be the effective width of the light beam, i.e.,about 2.5 mm. In the present embodiment, part of the perimeter of thearea of the interface between element 118 and the surface of plate 116is masked by a coating of opaque black chrome (not shown) to limit straylight within the system. Other non-functional areas of the back of plate116 may similarly be masked with the opaque coating, if desired.

The light source 122 provides, with reference to the silicon diodephotocells, a wavelength range of between about 600 1,000 nm. Anestimated average effective wavelength, A, of 850 nm serves quite wellfor initially computing the optimum spacing, 2, between grating elements111, 113 and minor final mechanical adjustments can readily be employedto obtain the actual spacing at which signals are of desired magnitude.

As earlier indicated, it is preferred, in measuring systems of thegeneral type which rely basically upon pulse counting, to provide foursignals in phase quadrature which may be utilized in eliminating DC.signal components as well as in distinguishing displacement direction.In the present embodiment, four such signals are generated directly asprimary light signals, thus eliminating the need for beam-dividingoptics or complicated electronic signaLseparating systems. Suchgeneration of four light signals is achieved in this preferredembodiment through the use of the multiple grid patterns shown in FIG.13.

Reticle grating element 11 1,'as indicated, bears on its face rulings ofsubstantially the same character as described with respect to element113. The pattern of the rulings, however, is such as to define fiveseparate grid patterns. The first grid 121 is centrally situated acrossthe face of plate 116 and underlies the effective interface area betweenelement 118 and plate 116 across the width dimension, W,.The transversepositioning of grid pattern 121 is selected arbitrarily. Each of theremaining grid patterns 101, 102, 103, 104 is situated in each of thequadrants of plate 116 and underlies substantially the whole of thefunctional area of the face of its associated photocell 201, 202, 203,204. While each of grid patterns 101-104 has a period, d, which issimilar to the grid periods of grating pattern 121 and that of element113, the respective transverse position of each is displaced withrespect to the next by an increment, d/4. For example, grid 102 lagsgrid 101, i.e., is displaced to the right in FIG. 13 with respect togrid 101, by one-half a line width. Similarly, grid 103 lags grid 102,etc. As will later become more apparent, each of grids 101-104 comprisesone of four grating elements which constitute the dual grating pairscharacteristic of the present invention, and thus the d/4 relativedisplacement of these grids effects a respective shift of 0/4 in theresulting directional preferences of each of four dual grating pairsystems formed with the 13 result that the signals derived at thephotocells 201-204 differ in phase by 90.

As noted, there are established in the present embodiment four separatedual grating pair systems. The first of these comprises as the inputpair, earlier designated as pair A, fixed grid 121 and the moveable gridof element 1 13. The second, or output pair, B, of the first systemcomprises the moveable grid of element 113 (due to beam reflection fromcoating 114) and fixed grid 101. The light intensity variationsgenerated by this dual pair system with movement of element 113 areintegrated by photocell 201 to provide a first electrical signal. Thesecond dual pair system comprises, in light flow sequence, grids 121,113, 113, 102 and photocell 202. The remaining two systems are similarlycomposed, varying in final grid and photocell elements; i.e., 103, 203and 104, 204. With movement of element 113 each of photocells 201-204generates the same substantially sinusoidal signal, yet these individualsignals differ sequentially in phase by 90.

The signals thus derived from the photocells may be employed in theusual manner known in the art to obtain a sine-cosine signal pair whichis distinctive of displacement direction as well as magnitude. Apreferred electronic system is of the push-pull common-modeeliminationtype and is shown schematically in FIG. 14. Photocells 201, 203 whichgenerate signals in 180 .phase opposition are connected in series tobias voltage source 141 and the resulting signal devoid of DC. componentis amplified at 145 and directed to counter 149, all according to wellknown procedures. The signals from photocells 202, 204 which arelikewise in 180 phase opposition are similarly employed in the circuitcomprising bias 143 and amplifier 147 to provide the second compositesignal, 90 out of phase with the first, as input to counter means 149.

The silicon diode photocells employed in this embodiment have aneffective transverse dimension, W,,, of about 1.6 mm, thus establishinga field of view, (FIG. of about for each such photocell. The effectiveperiod, 6, of directional preferences for this systern being about 3",it can be seen that each photocell integrates a light signal (FIG. 6)which comprises about 5 cycles. The resulting intensity of these lightsignals is of such a relatively high magnitude, particularly incomparison with viewing-field-limiting systems, that the system caneffectively operate on very little power as expended in the light sourceand amplifiers.

The described embodiment of the invention, when employed in a linealmeasuring system, provides displacement discrimination to about 0.002mm. In a comparable system employed for indicating angular displacement,as in a transit or theodolite, the moveable element 113 takes the formof a circular plate mounted for rotation upon a central axis' and therulings are radially disposed. Such a system having a grid' period, d,of about 0.0l6 and having the reticle grating element 111 located atabout mm from the center of circle plate 113 provides angulardiscrimination to about 0.002.

The foregoing embodiment has been presented for the purpose ofillustration and should not be taken to limit the scope of the presentinvention. It will be apparent that such embodiment is capable of manyvariations and modifications which are likewise to be included withinthe scope of the present invention as set forth in the appended claims.

What is claimed is:

1. A method of creating an electrical signal varying as a function ofphysical displacement which comprises:

a. arranging a light source and two pairs of amplitude grating patternsin sequence along an optical axis, the grid rulings comprising saidgrating patterns being substantially parallel and in parallel planesdisposed substantially perpendicular to said axis,

the grating patterns comprising each of said grating pattern pairs beingspaced apart by a distance substantially equal to (f /A; where d is thegrating pattern period and A is the effective wavelength of light fromsaid source, said spacing thereby establishing a series of directionallight transmission preferences varying cyclically in a directiontransverse to said grid rulings;

b. directing a beam of substantially omnidirectional light from saidsource through said two pairs of grating patterns along saidoptical axisto incidence upon photoelectric means capable of providing an electricalsignal varying as a function of the intensity of light from said sourceincident thereupon, said photoelectric means having a field of view withrespect to said light source inclusive of a plurality of cycles in saiddirectional light transmission preference series; and

c. displacing at least one of said grating patterns with respect to itspair-mate grating pattern in a direction transverse to said gridrulings, thereby effecting a variation in the intensity of the lighttransmitted through said two pairs of grating patterns and incident uponsaid photoelectric means as a function of the displacement of said atleastone grating pattern.

2. A method of determining incremental displacement between a pair ofobjects which comprises:

a. arranging a light source and two pairs of amplitude grating patternsin sequence along an optical axis, the grid rulings comprising saidgrating patterns being substantially parallel and in parallel planesdisposed substantially perpendicular to said axis, the grating patternscomprising each of said grating I pattern pairs being spaced apart by adistance substantially equal to d' lk; where d is the grating pat ternperiod and A is the effective wavelength of light from said source, saidspacing thereby establishing a series of directional light transmissionpreferences varying cyclically in a direction transverse to said gridrulings, at least one of said grating patterns being coupled with atleast one of said pair of objects insuch a manner as to effect arelative movement between said at least one grating pattern and itspair-mate grating pattern in a direction transverse to said grid rulingsas a function of said incremental displacement between said pair ofobjects;

b. directing a beam of substantially omnidirectional light from saidsource along said optical axis and through said grating pattern pairs toincidence upon photoelectric means capable of providing an electricalsignal varying as a function of the intensity of light from said sourceincident thereupon, said photoelectric means having a field of view withrespect to said light source inclusive of a plurality of cycles in saiddirectional light transmission preference series;

c. connecting said photoelectric means as electrical signal input sourcein circuit with electronic means capable of indicating displacement as afunction of the cycling of an electrical input signal; and

d. causing said incremental displacement between said object pair andcoincidentally said relative transverse movement between said pair ofgrating patterns; thereby effecting a cycling variation in the intensityof light transmitted by said two pairs of grating patterns and incidentupon said photoelectric means, and providing said cycling electricalinput signal indicative of said incremental displacement.

3. A device for providing a variation in an electrical signal as afunction of physical displacement which comprises:

a. a source of substantially omnidirectional light;

b. photoelectric means capable of providing an electrical signal varyingas a function of the intensity of light from said source incidentthereupon, said photoelectric means being disposed with respect to saidlight source so as to receive said omnidirectional light therefrom alonga path establishing an optical axis in said device; and

c. at least two pairs of amplitude grating patterns disposed in sequencealong said optical axis, the grid rulings comprising said gratingpatterns being substantially parallel and in parallel planes disposedsubstantially perpendicular to said axis, the grating patternscomprising each of said grating pattern pairs being spaced apart by adistance substantially equal to (i /A; where d is the grating patternperiod and A is the effective wavelength of light from said source, saidspacing thereby establishing a series of directional light transmissionpreferences varying cyclically in a direction transverse to said gridrulings;

d. said photoelectric means having a field of view with respect to saidlight source inclusive of a plurality of cycles in said directionallight transmission preference series;

e. at least one of said pairs of grating patterns being arranged forrelative movement therebetween in a direction transverse to the gridrulings of said grating patterns, thereby to effect said physicaldisplacement.

4. A device according to claim 3 which further comprises means wherebysaid optical axis is folded, and wherein said two pairs of gratingpatterns comprise a single pair of grating patterns which, by virtue ofsaid axis folding, is disposed at least once in each leg of said foldedaxis.

5. A device according to claim 3 wherein each of said grating patternsis carried by a separate transparent support and wherein one of saidsupports is arranged for movement in said transverse direction.

6. A device according to claim 3 wherein said two pairs of gratingpatterns respectively comprise, in sequence along said optical axis,first and second grating patterns carried on separate transparentsupports, and third and fourth grating patterns carried on separatetransparent supports, and wherein said supports are arranged forcoincident relative transverse movement in such a manner that thedirection of relative movement of said first grating pattern withrespect to said second grating pattern is opposite to the direction ofrelative movement of said third grating pattern with respect to saidfourth grating pattern.

7. A device according to claim 6 wherein said first grating pattern iscarried on a first support, said second and third grating patterns arecarried, respectively, on opposite faces of a second support, and saidfourth grating pattern is carried on a third support.

8. A device according to claim 6 which further comprises reflectivemeans whereby said optical axis is folded, and wherein, by virtue ofsaid axis folding, a grating pattern carried by one of a pair of saidsupports comprises both of said first and fourth grating patterns and agrating pattern carried by the other one of said pair of supportscomprises both of said second and third grating patterns.

9. A device according to claim 6 wherein said first and fourth gratingpatterns are carried by a firsttransparent support and a separategrating pattern is carried by a second transparent support, said devicefurther comprising means whereby said optical axis is folded and causedto double-pass said second support, said separate grating patternthereby comprising said second and third grating patterns, at least oneof said first and second supports being arranged to provide relativeopposite transverse movement between said supports.

10. A displacement-indicating device comprising:

a. a source of substantially omnidirectional light;

b. photoelectric means capable of providing an electrical signal varyingas a function of the intensity of light from said source incidentthereupon, said photoelectric means being disposed with respect to saidlight source so as to receive said omnidirectional light therefrom alonga-path establishing an optical axis in said device;

. a plurality of amplitude grating patterns comprising, in sequencealong said optical axis:

1. a first amplitude grating pattern situated in a plane substantiallyperpendicular to said axis;

2. a second amplitude grating pattern situated in a plane substantiallyperpendicular to said axis;

3. a third amplitude grating pattern situated in a plane substantiallyperpendicular to said axis; and

4. a fourth amplitude grating pattern situated in a plane substantiallyperpendicular to said axis;

5. the grid rulings of all said grating patterns being substantiallyparallel as viewed along said optical axis;

6. said first and second grating patterns, and said third and fourthgrating patterns, respectively, being spaced apart by a distancesubstantially equal to d /A; where d is the grating pattern period and Ais the effective wavelength of light from said source, said spacingthereby establishing a series of directional light transmissionpreferences varying cyclically in a direction transverse to said gridrulings;

d. said photoelectric means having a field of view with respect to saidlight source inclusive of a plurality of cycles in said directionallight transmission preference series;

e. at least one of said'gratin g patterns being arranged for movement'inits plane in a direction transverse to its grid rulings, thereby toeffect the displacement to be indicated; and

f. electronic means in circuit with said photoelectric means andresponsive to electrical signals provided by said photoelectric means toprovide an indication of displacement. I i

11. A device according to claim 10 wherein said first and fourth gratingpatterns are carried by a first transparent support and a separategrating pattern is carried by a second transparent support, said devicefurther comprising means whereby said optical axis is folded and causedto double-pass said second support, said separate grating patternthereby comprising said second and third grating patterns, at least oneof said first and second supports being arrangedto provide relativeopposite transverse movement between said supports.

12. A device according to claim 10 wherein a plurality of said gratingpatterns are arranged for coincident transverse movement in such amanner as to effect relative movement of said first grating pattern withrespect to said second grating pattern in a direction opposite to therelative movement of said third grating pattern with respect to saidfourth grating pattern.

13. A device according to claim 12 wherein said second and said thirdgrating patterns are carried in common by a single transparent support,and wherein said support is arranged for said transverse movement,thereby to effect said opposite directions of relative grating patternmovement.

14. A device according to claim 10 wherein said first grating pattern iscarried by a first transparent support and said second grating patternis carried by a second transparent support, and wherein said meansestablishing said optical axis comprises means whereby said optical axisis folded and caused to double-pass said supports, said second and firstgrating patterns thereby comprising, respectively, said third and fourthgrating patterns, at least one of said first and second supports beingarranged to provide relative opposite transverse movement between saidsupports.

15. A device according to claim 14 wherein said optical axis foldingmeans comprises a reflective surface situated subsequent to said secondgrating pattern in said sequence along said optical axis and disposedsubstantially perpendicular to said axis.

16. A device according to claim 10 wherein:

a. at least one of said grating patterns comprises a plurality ofregions wherein the grid rulings comprising each of said grating patternregions are transversely displaced with respect to thegrid rulingscomprising each other grating pattern of said plurality of regions by afractional increment of the period of said at least one grating pattern;and

b. said photoelectric means comprises a plurality of photocells, each ofsaid photocells being arranged to respectively receive light transmittedby a different one of said plurality of grating pattern regions;

whereby a plurality of phase-displaced electrical signals are generatedby said photoelectric means in response to light transmitted throughsaid plurality of amplitude grating patterns along said optical axis.

17. A device according to claim 16 wherein said at least one gratingpattern comprises four grating pattern regions, and wherein said regionsare displaced sequentially by an increment substantially equal toonequarter of said grating pattern period, thereby providingphase-quadrature displacement in four signals generated by saidphotoelectric means.

1. A method of creating an electrical signal varying as a function ofphysical displacement which comprises: a. arranging a light source andtwo pairs of amplitude grating patterns in sequence along an opticalaxis, the grid rulings comprising said grating patterns beingsubstantially parallel and in parallel planes disposed substantiallyperpendicular to said axis, the grating patterns comprising each of saidgrating pattern pairs being spaced apart by a distance substantiallyequal to d2/ lambda ; where d is the grating pattern period and lambdais the effective wavelength of light from said source, said spacingthereby establishing a series of directional light transmissionpreferences varying cyclically in a direction transverse to said gridrulings; b. directing a beam of substantially omnidirectional light fromsaid source through said two pairs of grating patterns along saidoptical axis to incidence upon photoelectric means capable of providingan electrical signal varying as a function of the intensity of lightfrom said source incident thereupon, said photoelectric means having afield of view with respect to said light source inclusive of a pluralityof cycles in said directional light transmission preference series; andc. displacing at least one of said grating patterns with respect to itspair-mate grating pattern in a direction transverse to said gridrulings, thereby effecting a variation in the intensity of the lighttransmitted through said two pairs of grating patterns and incident uponsaid photoelectric means as a function of the displacement of said atleast one grating pattern.
 2. a second amplitude grating patternsituated in a plane substantially perpendicular to said axis;
 2. Amethod of determining incremental displacement between a pair of objectswhich comprises: a. arranging a light source and two pairs of amplitudegrating patterns in sequence along an optical axis, the grid rulingscomprising said grating patterns being substantially parallel and inparallel planes disposed substantially perpendicular to said axis, thegrating patterns comprising each of said grating pattern pairs beingspaced apart by a distance substantially equal to d2/ lambda ; where dis the grating pattern period and lambda is the effective wavelength oflight from said source, said spacing thereby establishing a series ofdirectional light transmission preferences varying cyclically in adirection transverse to said grid rulings, at least one of said gratingpatterns being coupled with at least one of said pair of objects in sucha manner as to effect a relative movement between said at least onegrating pattern and its pair-mate grating pattern in a directiontransverse to said grid rulings as a function of said incrementaldisplacement between said pair of objects; b. directing a beam ofsubstantially omnidirectional light from said source along said opticalaxis and through said grating pattern pairs to incidence uponphotoelectric means capable of providing an electrical signal varying asa function of the intensity of light from said source incidentthereupon, said photoelectric means having a field of view with respectto said light source inclusive of a plurality of cycles in saiddirectional light transmission preference series; c. connecting saidphotoelectric means as electrical signal input source in circuit withelectronic means capable of indicating displacement as a function of thecycling of an electrical input signal; and d. causing said incrementaldisplacement between said object pair and coincidentally said relativetransverse movement between said pair of grating patterns; therebyeffecting a cycling variation in the intensity of light transmitTed bysaid two pairs of grating patterns and incident upon said photoelectricmeans, and providing said cycling electrical input signal indicative ofsaid incremental displacement.
 3. A device for providing a variation inan electrical signal as a function of physical displacement whichcomprises: a. a source of substantially omnidirectional light; b.photoelectric means capable of providing an electrical signal varying asa function of the intensity of light from said source incidentthereupon, said photoelectric means being disposed with respect to saidlight source so as to receive said omnidirectional light therefrom alonga path establishing an optical axis in said device; and c. at least twopairs of amplitude grating patterns disposed in sequence along saidoptical axis, the grid rulings comprising said grating patterns beingsubstantially parallel and in parallel planes disposed substantiallyperpendicular to said axis, the grating patterns comprising each of saidgrating pattern pairs being spaced apart by a distance substantiallyequal to d2/ lambda ; where d is the grating pattern period and lambdais the effective wavelength of light from said source, said spacingthereby establishing a series of directional light transmissionpreferences varying cyclically in a direction transverse to said gridrulings; d. said photoelectric means having a field of view with respectto said light source inclusive of a plurality of cycles in saiddirectional light transmission preference series; e. at least one ofsaid pairs of grating patterns being arranged for relative movementtherebetween in a direction transverse to the grid rulings of saidgrating patterns, thereby to effect said physical displacement.
 3. athird amplitude grating pattern situated in a plane substantiallyperpendicular to said axis; and
 4. a fourth amplitude grating patternsituated in a plane substantially perpendicular to said axis;
 4. Adevice according to claim 3 which further comprises means whereby saidoptical axis is folded, and wherein said two pairs of grating patternscomprise a single pair of grating patterns which, by virtue of said axisfolding, is disposed at least once in each leg of said folded axis.
 5. Adevice according to claim 3 wherein each of said grating patterns iscarried by a separate transparent support and wherein one of saidsupports is arranged for movement in said transverse direction.
 5. thegrid rulings of all said grating patterns being substantially parallelas viewed along said optical axis;
 6. said first and second gratingpatterns, and said third and fourth grating patterns, respectively,being spaced apart by a distance substantially equal to d2/ lambda ;where d is the grating pattern period and lambda is the effectivewavelength of light from said source, said spacing thereby establishinga series of directional light transmission preferences varyingcyclically in a direction transverse to said grid rulings; d. saidphotoelectric means having a field of view with respect to said lightsource inclusive of a plurality of cycles in said directional lighttransmission preference series; e. at least one of said grating patternsbeing arranged for movement in its plane in a direction transverse toits grid rulings, thereby to effect the displacement to be indicated;and f. electronic means in circuit with said photoelectric means andresponsive to electrical signals provided by said photoelectric means toprovide an indication of displacement.
 6. A device according to claim 3wherein said two pairs of grating patterns respectively comprise, insequence along said optical axis, first and second grating patternscarried on separate transparent supports, and third and fourth gratingpatterns carried on separate transparent supports, and wherein saidsupports are arranged for coincident relative transverse movement insuch a manner that the direction of relative movement of said firstgrating pattern with respect to said second grating pattern is oppositeto the direction of relative movement of said third grating pattern withrespect to said fourth grating pattern.
 7. A device according to claim 6wherein said first grating pattern is carried on a first support, saidsecond and third grating patterns are carried, respectively, on oppositefaces of a second support, and said fourth grating pattern is carried ona third support.
 8. A device according to claim 6 which furthercomprises reflective means whereby said optical axis is folded, andwherein, by virtue of said axis folding, a grating pattern carried byone of a pair of said supports comprises both of said first and fourthgrating patterns and a grating pattern carried by the other one of saidpair of supports comprises both of said second and third gratingpatterns.
 9. A device according to claim 6 wherein said first and fourthgrating patterns are carried by a first transparent support and aseparate grating pattern is carried by a second transparent support,said device further comprising means whereby said optical axis is foldedand caused to double-pass said second support, said separate gratingpattern thereby comprising said second and third grating patterns, atleast one of said first and second supports beIng arranged to providerelative opposite transverse movement between said supports.
 10. Adisplacement-indicating device comprising: a. a source of substantiallyomnidirectional light; b. photoelectric means capable of providing anelectrical signal varying as a function of the intensity of light fromsaid source incident thereupon, said photoelectric means being disposedwith respect to said light source so as to receive said omnidirectionallight therefrom along a path establishing an optical axis in saiddevice; c. a plurality of amplitude grating patterns comprising, insequence along said optical axis:
 11. A device according to claim 10wherein said first and fourth grating patterns are carried by a firsttransparent support and a separate grating pattern is carried by asecond transparent support, said device further comprising means wherebysaid optical axis is folded and caused to double-pass said secondsupport, said separate grating pattern thereby comprising said secondand third grating patterns, at least one of said first and secondsupports being arranged to provide relative opposite transverse movementbetween said supports.
 12. A device according to claim 10 wherein aplurality of said grating patterns are arranged for coincidenttransverse movement in such a manner as to effect relative movement ofsaid first grating pattern with respect to said second grating patternin a direction opposite to the relative movement of said third gratingpattern with respect to said fourth grating pattern.
 13. A deviceaccording to claim 12 wherein said second and said third gratingpatterns are carried in common by a single transparent support, andwherein said support is arranged for said transverse movement, therebyto effect said opposite directions of relative grating pattern movement.14. A device according to claim 10 wherein said first grating pattern iscarried by a first transparent support and said second grating patternis carried by a second transparent support, and wherein said meansestablishing said optical axis comprises means whereby said optical axisis folded and caused to double-pass said supports, said second and firstgrating patterns thereby comprising, respectively, said third and fourthgrating patterns, at least one of said first and second supports beingarranged to provide relative opposite transverse movement between saidsupports.
 15. A device according to claim 14 wherein said optical axisfolding means comprises a reflective surface situated subsequent to saidsecond grating pattern in said sequence along said optical axis anddisposed substantially perpendicular to said axis.
 16. A deviceaccording to claim 10 wherein: a. at least one of said grating patternscomprises a plurality of regions wherein the grid rulings comprisingeach of said grating pattern regions are transversely displaced withrespect to the grid rulings comprising each other grating pattern ofsaid plurality of regions by a fractional increment of the period ofsaid at least one grating pattern; and b. said photoelectric meanscomprises a plurality of photocells, each of said photocells beingarranged to respectively receive light transmitted by a different one ofsaid plurality of grating pattern regions; whereby a plurality ofphase-displaced electrical signals are generated by said photoelectricmeans in response to light transmitted through said plurality ofamplitude grating patterns along said optical axis.
 17. A deviceaccording to claim 16 wherein said at least one grating patterncomprises four grating pattern regions, and wherein said regions aredisplaced sequentially by an increment substantially equal toone-quarter of said grating pattern period, thereby providingphase-quadrature displacement in four signals generated by saidphotoelectric means.